Feature: eliminate "pre-FCP" procedures for merge/close/postpone

[nikomatsakis]

As discussed in rust-lang/wg-governance#38, I would like to streamline our rfcbot procedures as follows:

First, eliminate the "pre-FCP" period for fcp merge. What this means is that when someone proposed "rfcbot fcp merge", we go immediately into the "final-comment-period". The final comment period lasts until all of the following have occurred:

• 10 days have elapsed.
• Sufficient team members have checked their boxes.
• There are no unresolved concerns.

Second, make close/postpone easier. In the case of close/postpone, we would also go immediately to final comment period, but with the following ending conditions:

• 10 days have elapsed.
• there are no unresolved concerns

In particular, we would not have "checkboxes" for close/postpone, and would instead just ping the team to give them a chance to raise concerns.

This is based on the following observations:

• Once an rfcbot fcp merge occurs, people treat it as though FCP has occurred and there is immediate discussion. It usually takes way longer to get all the team members to check their boxes, and the final 10 days is almost always empty space.
• Similarly, closing/postponing is awfully hard and it's a lot of busy work to get folks to check their boxes just to keep the status quo.